The body of an aircraft and the shock-absorbers of its landing-gear are designed to absorb the kinetic energy stored by the total moving mass in the event of a crash landing. In particular, for a helicopter, the vertical landing speed may be as much as 12 meters per second (m/s). The landing-gear shock-absorbers then absorb a portion of the kinetic energy, and the remainder is absorbed by the helicopter body deforming or breaking up in part.
For combination actuator/shock-absorbers specially designed for such a situation, reference may be made to the following documents that come from the Applicant: EP-A-0 564 325, EP-A-0 564 324, EP-A-0 051 506, and EP-A-0 014 660, and also to the following documents: EP-A-0 275 735, EP-A-0 115 435, and EP-A-0 072 323.
In spite of the limit on acceleration (in a vertical direction) obtained by means of the landing-gear, it can still happen that the acceleration to which the transported system is subjected exceeds a critical threshold (e.g. greater than 20 g, which gives rise,to major injury to people being transported), either because the limitation on acceleration is insufficient, or else because of the helicopter moving in directions that are relatively random.
In particular, the acceleration of the seats can exceed the critical threshold for the people sitting on them. Similarly, the acceleration to which slinging is subjected may exceed the critical threshold for the objects stowed in the aircraft body.
The commonly used solution then consists in using shock-absorbers for connecting the objects concerned (seats, transported loads) to the structure of the aircraft. In particular, peak-limiting shock-absorbers are presently used beneath the seats, with a shock-absorber being disposed between each seat and the body of the aircraft. For example, devices are known that rely on plastic deformation of a tube into which a larger-diameter ball is thrust (each vertical seat fastening is then supported on a ball disposed in the neck of an opening to a tube which is designed to deform plastically as it absorbs energy), and although such a system is indeed cheap, it is not very accurate (accuracy being a function of manufacturing tolerances and of the materials used).
In general, when using shock-absorbers, the acceleration of the transported system depends necessarily on its mass and on changes in the speed of the supports to which it is attached. It is therefore very difficult to be accurate, given that the weight of a person (plus equipment in a military helicopter) can vary to a large extent, such that, for given acceleration, the forces involved can vary over a range of two to one. This can naturally be even more significant with multi-person seats that may be occupied by one person or by several.